The present invention relates to electronic analog circuits, and more particularly, to transimpedance amplifiers.
With increasing data rates in electronic systems, it is expected that optical interconnects (optical fibers) may in the near future replace wire interconnects at the board-to-board and chip-to-chip level. For example, a computer system such as that illustrated in FIG. 1 may comprise one or more boards 102 and memory hierarchy 104 that exchange data packets over optical interconnects 106. These packets may be routed via switch 108, or perhaps the various integrated circuits may be directly connected to one another. Each board 102 may comprise one or more microprocessors.
In many applications, a photo-detector provides an electrical signal indicative of a received optical signal. A simplified small-signal model for a photo-detector is a small-signal current source, where the small-signal current is representative of the received optical signal. Transimpedance amplifiers provide a small-signal output voltage signal in response to a small-signal input current signal. Many transimpedance amplifiers used in optoelectronic telecommunication applications employ the two popular designs shown in FIGS. 2a and 2b. 
The photo-detector in FIGS. 2a and 2b is modeled as small-signal current source 202 and small-signal parasitic capacitor 204. The transimpedance amplifier of FIG. 2a is a two stage, common-source, common-drain amplifier. The first stage comprises common-source nMOSFET (n-Metal-Oxide-Semiconductor-Field-Effect-Transistor) 206 and load pMOSFET 208. The second stage comprises common-drain nMOSFET 210 and load pMOSFET 212. Resistor 214 provides negative feedback. The transimpedance amplifier of FIG. 2b is a single stage, common-gate amplifier, where the single stage comprises common-gate nMOSFET 218, with nMOSFET 216 and pMOSFET 220 providing bias current and active loads to common-gate nMOSFET 218.
In telecommunication applications, the received optical signals are typically very small due to attenuation in optical fibers, which may be hundreds of kilometers long. Consequently, a primary goal for transimpedance amplifiers for long haul communications is to provide high transimpedance with low noise amplification, while attaining as large a bandwidth as practical.
However, at the board-to-board and chip-to-chip level, such as the computer system of FIG. 1, attenuation is relatively negligible, and the received signals are typically orders of magnitude larger than for the case of long haul optical communication systems. In such short haul optical applications, transimpedance amplifiers may be integrated with other circuits on an integrated circuit die, such as input-output chips on boards 102 or switch 108, and perhaps on a microprocessor itself. Accordingly, for such applications, designing for large bandwidth while minimizing power plays a critical role in the design of transimpedance amplifiers. Under these criteria, the amplifiers in FIGS. 2a and 2b may contain several drawbacks.
One drawback is that each stage in the amplifier of FIG. 2a requires a relatively large DC bias current. Furthermore, the two load transistors 208 and 212 are each biased at bias voltages VB1 and VB2, respectively. To avoid bias voltage coupling, two bias circuits may be needed to bias the load transistors. Consequently, power consumption for the circuit of FIG. 2a may be too high. Another drawback is that common-drain nMOSFET 210 may cause a 30% to 50% drop in transimpedance from the first stage output to the second stage output.
The amplifier of FIG. 2b contains only one stage, but transistors 216, 218, and 220 require three bias voltages VB1, VB2, and VB3, respectively. Again, to avoid bias voltage coupling, three separate bias circuits may be needed for the amplifier of FIG. 2b, thus contributing to power consumption. Furthermore, a problem shared by amplifiers requiring one or more constant bias voltages is that generating a constant bias voltage with good power supply noise rejection is considered a difficult problem.
Consequently, transimpedance amplifiers that are commonly used for long haul communications may not be suitable for short haul optical communications, such as computer systems, where power consumption may be an issue.